Bloodraven/Bran in actuality, since they can effectively time-travel and control most living things in Westeros. But I have a feeling that they will be fairly laissez-faire and not interfere too greatly to the goings-on of Westeros.

In terms of everyone else, I'd have to go with Euron. He has a formidable force at his disposible, and seems to be a sole decision-maker. He is eating away at the soft underbelly of Westeros but he could flex his muscles in any number of ways, and I feel his Dragonbinder is an ace in his sleeve (given that "Euron's gifts are poisoned"). However, that's not to say that a change in tides or a clever allegiance couldn't spell the end of him shortly.

Littlefinger is de jure lord of the Riverlands, de facto Lord of the Vale, and has a great deal of influence over one of the few remaining Starks. It'll probably all come crashing down at some point, but for the moment he's looking pretty powerful.

By the end? I think we can argue Mace Tyrell as far as Westerosi characters go.

-Hand of the King(and possibly soon to be regent)
-Leads the largest army in the continent
-House Tyrell is a rich house and by marriage he has connections to House's Redwyne and Hightower who are also insanely rich
-Has 2 of his goons on the small council and now that Pycelle and Kevan are dead he can add 1 or 2 more his men(like Mathis Rowan or anyone else) to the council
-Tommen is a puppet to him and the only thing that stands in his way is a powerless Cersei

Thank you for mentioning this, they or te Iron Bank are probably the most powerful organization in ASOIAF, thats why i put the Sealord of Braavos up there because im pretty sure he and the IB have pull with the Faceless Men

Very hard to tell at this point, without omniscience. But my vote goes to Varys. He is able to pull all the strings he needs from the background. But with all the things yet to come, well, I'm not betting any hard money on anyone.

The most powerful person in Westeros right now is the High Sparrow-- he can put Cersie in check, even Stannis couldn't do that.

I agree. I assume we are talking political power.

I also think that Dany probably has the strongest single army, though she doesnt have any real allies.

As far as political power in Westeros, Jon has quite a large following with the Wildlngs and the NW if he survives, and Littlefinger is just a few moves away from commanding the Vale which has not been toached by war, and could gain the North if he can convince (trick) Sansa into a political marriage and revealing who she is...

Lots of equal parties in Westeros right now, but I give the nod to Littlefinger because I can tell that he has been plotting his rise for a long time.